fix Pool to remove possibility of "leaking" connections (#84)

* fix `Pool` to reduce possibility of "leaking" connections

now uses RAII guards to control `SharedPool::size`

* add smoke test for `Pool` to both Postgres and MySQL tests

add `Pool::is_closed()`

* fix documentation re: pool

* refactor pool implementation to not use futures oneshot channels

https://github.com/launchbadge/sqlx/pull/84#issuecomment-580476223

* run cargo fmt

* Pool: remove superfluous guard struct, document some internal methods

sqlx-core/src/postgres/connection.rs

@@ -18,9 +18,9 @@ use crate::postgres::PgError;
 use crate::url::Url;
 use crate::Result;
 
-/// An asynchronous connection to a [Postgres] database.
+/// An asynchronous connection to a [Postgres][super::Postgres] database.
 ///
-/// The connection string expected by [Connection::open] should be a PostgreSQL connection
+/// The connection string expected by [Connect::connect] should be a PostgreSQL connection
 /// string, as documented at
 /// <https://www.postgresql.org/docs/12/libpq-connect.html#LIBPQ-CONNSTRING>
 ///

sqlx-core/src/postgres/mod.rs

@@ -16,5 +16,5 @@ mod protocol;
 mod row;
 mod types;
 
-/// An alias for [`Pool`], specialized for **Postgres**.
+/// An alias for [`Pool`][crate::Pool], specialized for **Postgres**.
 pub type PgPool = super::Pool<PgConnection>;